Hi, Elena
Why *do English speakers say 'partner' instead of 'girlfriend/boyfriend'?
Couples may say this because they are unmarried, and they feel that they're too mature to use the terms 'girlfriend/boyfriend'. Instead of 'partner', you may also hear the term 'significant other'.
Some married couples use the term 'partner', but it's not common. In fact, I've never referred to my husband as my 'partner' when speaking about him to another person. When speaking to my husband, I've said, "We're partners in this!" However, I wasn't referring to our marriage; I was talking about a joint venture of some sort.
I hear 'partner' quite often among same-sex couples.
Cambridge Dictionary:
*Partner: a person’s partner can also be the person to whom someone is married, or to whom the person is not married but with whom they have a close relationship.
*Significant other: a person with whom someone has a romantic or sexual relationship that has lasted for some time and that is likely to last longer.
I hope this helps!